1. Field of the Invention
The present invention relates to a method for separating a mixed gas comprising oxygen and chlorine. The invention also relates to a system for separating a mixed gas comprising oxygen and chlorine. The inventive method and system have particular application in ore processing, for example, in the treatment of an exhaust gas from an iron chloride oxidation reactor.
2. Description of the Related Art
In iron ore processing, iron chlorides are oxygenated during roasting, in the presence of excess oxygen, yielding iron oxides, chlorine and oxygen, as well as byproducts of the reaction. The chlorine and oxygen are generally in the form of a mixed gas.
Attempts have been made to recycle the chlorine gas removed from the oxidation reactor to the ore processing reactor as part of an impure stream of byproducts including various gases. It has also been proposed to remove a relatively pure product stream of chlorine gas from the mixed gas effluent, while recycling the remainder of the effluent to the processing reactor.
To the present inventor's knowledge, however, there has been no attempt to recapture both the excess oxygen from oxidation of the iron chlorides and the chlorine thus produced as relatively pure product streams. In particular, there has not heretofore been a technique of separating a mixed gas containing oxygen and chlorine by cryogenic processing.
In developing the present invention, the inventor considered various processes, which, for one reason or another, were deemed not optimal for accomplishing the separation of oxygen and chlorine from a mixed gas. For example, an effluent gas containing chlorine and oxygen may be separated into its components by passing it through a suitable membrane. However, there are several problems with this method. Membrane separation of chlorine and oxygen is typically poor because the permeation characteristics of oxygen and chlorine are often very similar. Further, even if a good separation could be achieved, chlorine can be highly corrosive to separation membranes, which are generally amide-based polymers containing chlorine in the polymer backbone. Therefore, relatively small amounts of chlorine would be expected to destroy the membrane, leading to high maintenance and material costs. In addition, as membrane-based gas separation processes are driven by partial pressure differentials, relatively high feed pressures are required to effect separation. Compression of the feed gas to nominally 100 to 150 psig would be relatively costly even if the separation technology was feasible.
Another possible separation technique is a solvent-based separation using a solvent, such as water or titanium tetrachloride (TiCl.sub.4), which can selectively absorb chlorine without absorbing much of the oxygen. However, TiCl.sub.4 represents an environmental hazard and cannot easily be disposed of after use. Further, like any solvent-based separation system, it is difficult to separate the desired product, whether oxygen or chlorine, from the solvent used to absorb that product.
The inventor has further considered the use of pressure swing adsorption for separation of a mixed gas containing oxygen and chlorine. For such a system to work, it would first be necessary to develop a suitable dry bed particle adsorbent system which could adsorb the chlorine while allowing oxygen to pass through without hindrance. One of the silica or molecular sieve products currently in use for oxygen separation may be useful. However, it is unknown how the silica or molecular sieve products would react to chlorine or how effective the oxygen/chlorine separation process would be. Apparently, extensive product research would be required to develop such a system.
Although use of both the solvent-based separation and pressure swing adsorption systems is thought to be possible, both systems require extensive capital and labor costs for development and execution. For the foregoing reasons, therefore, membrane, solvent-based and pressure swing separation techniques are not currently desirable for the separation of oxygen and chlorine from a mixed gas.
Therefore, it is an object of the present invention to provide a novel method and system for separating a mixed gas comprising oxygen and chlorine into an oxygen-enriched gas fraction and a chlorine-rich liquid fraction. The method and system are designed to meet the requirements of the ore processing industry and to overcome the disadvantages and difficulties associated with the related art.
The inventive method makes possible effective separation of oxygen from chlorine in a mixed gas in an economical manner, since both the oxygen-enriched gas fraction and the chlorine-rich liquid fraction which are produced can be recycled for use in various applications. For example, where the mixed gas is an effluent from an ore processing plant, the oxygen-enriched gas fraction can be recycled back to an oxidation reactor, while the chlorine-rich liquid fraction can be used elsewhere in the ore treatment process. Alternatively, either fraction can be stored for later use in the same or other processes.
It is a further object of the present invention to provide a novel system for separating a mixed gas comprising oxygen and chlorine, wherein the system can be used to practice the inventive method.
Other objects and advantages of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings, and claims appended hereto.